Bolivia’s rapidly diminishing Chacaltaya glacier has been widely used as a symbol of Anthropogenic Global Warming (1). However, it is an unfortunate choice of symbol, because the retreat of this specific glacier is demonstrably not due to increasing temperatures caused by CO₂ emissions.

These observed negative temperature trends in the Bolivian highlands beg two questions:

1. How can the falling temperatures in the Bolivian highlands be reconciled with the visibly diminishing glaciers?

2. Why are temperatures falling, when they are supposed to be increasing?

The answers to these questions are quite complex and involve many different factors.

First, it is important to understand that changes in glaciers do not only depend on temperatures, but also on precipitation, cloud cover, relative humidity and the intensity of solar irradiation. And in the case of temperatures, summer temperatures (rainy season) are more important than winter temperatures (dry season), and daytime temperatures are more important than night time temperatures.

In Figure 2 above, we see that winter temperatures have fallen more strongly than summer temperatures (the average winter anomaly is -0.9ºC while average summer anomaly is -0.4ºC). Since winter is the dry season in this region, colder winter temperatures will have little effect on the glaciers because temperatures are already well below freezing. In contrast, warmer summer temperatures can have a dramatic effect. It was the unusually hot and dry summer of 1998 (the Mega-El Niño) which caused the permanent closing of the Chacaltaya ski-resort, and the four consecutive warm summers of 2002, 2003, 2004 and 2005 almost completely eliminated the glacier.

A meteorologist from the Hadley Climate Research Centre suggested to me that the likely explanation for the observed decrease in temperatures in the Bolivian highlands is a decrease in low level clouds. With fewer low level clouds, night time temperatures would tend to fall substantially (due to increased outgoing infra-red radiation), which would pull down average temperatures. During the day, fewer clouds would have a positive effect on temperatures, increase solar irradiation reaching the ice, and decrease air humidity and precipitation, all of which would contribute to speed up melting, despite stronger night time cooling.

I could not find specific data on cloud cover in the Bolivian highlands, but worldwide low level cloud cover has clearly decreased since about 1987 (see Figure 3). According to NASA/GISS the decrease is particularly strong in the tropics, including Bolivia (3), so the meteorologist’s hunch is likely to be correct.

Figure 3: Global low level cloud cover, 1983-2007

 

It is ironic that the melting Chacaltaya glacier has become such an important symbol of the AGW theory, when in fact the evidence from Chacaltaya seems to refute this theory. (In contrast, the evidence from Chacaltaya is fully consistent with Svensmark’s cosmic ray theory (5), but that is another story).